The present invention relates to a steam generator in general, and more particularly to a steam generator of the type in which a bundle of steam-generating tubes extends through a fluidized-bed combustion chamber.
Steam generators having bundles of tubes extend through a combustion chamber in which the tubes are heated, so as to convert water flowing through the tubes into steam, are well known in the prior art (c.f. Chemical Engineers Handbook, 5th Edition, Chapter 11), as are fluidized-bed chambers (Ibid, Chapter 20, pages 64-74). The principle of using a fluidized-bed combustion chamber for steam generation is to obtain as uniform as possible a heat exchange with the tubes and their contents. For this purpose the surface areas in the tube bundle--which is located within the fluidized-bed layer--are so dimensioned that a constant layer temperature is obtained. This is the optimum operating temperature and it is desired to maintain it, even if the equipment operates only at partial capacity. For this purpose, therefore, less heat is withdrawn from the fluidized bed, which can be obtained by lesser cooling of the steam-generating tubes (smaller water flow). A problem with this is that it results in the development of tube-wall temperatures which are impermissibly high for the materials conventionally used for making such tubes.
One solution to the problem, which has been proposed in the prior art, is to make the vertical height of the fluidized-bed variable. The tubing extending into the fluidized-bed is so inclined to the vertical and to the horizontal that the length of the tubing which extends into the fluidized-bed depends upon the height (or depth) of the fluidized-bed. If the depth of the fluidized-bed changes, the thickness and therefore its function are influenced. However, when the equipment operates only at partial capacity it is no longer possible to optimally select the depth of the fluidized-bed, or else the range within regulation of the heat exchange can be effected is very limited.
Another proposal suggests to subdivide the fluidized-bed into several sections, each of which is provided with a heat exchange arrangement. When the equipment is to be operated at partial capacity, some of these sections are then simply not used. However, with this construction the regulation can be effected only by statures, i.e. a continuous adjustment is not possible.